Specific activity of phosphorus in mycorrhizal and non-mycorrhizal plants in relation to the availability of phosphorus to plants

Phosphate was allowed to react with a soil to which iron hydroxide had been added. The P was then labelled by a subsequent addition of ³²P. Soil P was extracted by 10 mm CaCl₂, 0.5 m NaHCO₃, and acid NH₄F solutions and the specific activity of P in the extracts was measured. Subterranean clover plants were grown both with and without a mycorrhizal fungus. Phosphorus contents and the specific activities of P in the plant shoots were determined. For mycorrhizal plants, adding iron hydroxide had no effect on the amount of P taken up, but for non-mycorrhizal plants it decreased the uptake. However there was no effect of iron hydroxide or of mycorrhizal infection on the specific activity of P in the plants. Adding iron hydroxide had no effect on the amount of P extracted by acid NH₄F, but decreased the P extracted by 10mm CaCl₂ and by 0.5 m NaHCO₃. The specific activity of P in the extracts was not affected by the addition of iron hydroxide and was the same for the three extractants. Further, the specific activity of P in all extractants was similar to that of P in both mycorrhizal and non-mycorrhizal plants. Thus differences in the availability of soil P to mycorrhizal and non-mycorrhizal plants and to the extractants were not reflected by differences in labelling. It therefore follows that lack of difference in specific activity between mycorrhizal and non-mycorrhizal plants does not eliminate the possibility that mycorrhizal plants can obtain P that was unavailable to non-mycorrhizal plants.